This invention relates to digital offset printer systems, and in particular to color printers having single-pass imaging systems that utilize multiple DMD (Digital Micro-mirror Device) devices for high speed image transfer operations.
In Variable Data Lithography (also referred to as Digital Lithography or Digital Offset) printing process begins with a fountain solution used to dampen a silicone imaging plate on an imaging drum. The fountain solution forms a film on the silicone plate that is on the order of about one (1) micron thick. The drum rotates to an exposure station where a high power laser imager is used to remove the fountain solution at the locations where the image pixels are to be formed. This forms a fountain solution based latent image. The drum then further rotates to a development station where lithographic-like ink is brought into contact with the fountain solution based latent image and ink develops onto the places where the laser has removed the fountain solution. The ink is hydrophobic. An ultra violet (UV) light may be applied so that photo-initiators in the ink may partially cure the ink to prepare it for high efficiency transfer to a print media such as paper. The drum then rotates to a transfer station where the ink is transferred to a printing media such as paper. The silicone plate is compliant, so an offset blanket is not used to aid transfer. UV light may be applied to the paper with ink to fully cure the ink on the paper. The ink is on the order of one (1) micron pile height on the paper.
The formation of the image on the printing plate is done with imaging modules each using a linear output high power infrared (IR) laser to illuminate a digital light projector (DLP) multi-mirror array, also referred to as the “DMD” (“Digital Micro-mirror Device” or “Digital Micromirror Device”) to guide a beam onto the surface of the printing plate. The mirror array is similar to what is commonly used in computer projectors and some televisions. The laser provides constant illumination to the micro-mirror array. The mirror array deflects individual mirrors to form the pixels on the image plane to pixel-wise evaporate the fountain solution on the silicone plate. If a pixel is not to be turned on, the mirrors for that pixel deflect such that the laser illumination for that pixel does not hit the silicone surface, but goes into a chilled light dump heat sink.
Multiple DMDs have been used in the formation of the image on the printing plate for various reasons such as to increase the brightness of the projected image, to enlarge the projected image, to reduce the duty cycle and thus reducing the potential for overheating for each DMD, and to increase the refresh rate of slower DMDs because each would only be responsible for a segment of the projected image. In order to form a cohesive image on the printing plate the use of mosaicking or stitching techniques are used to cover the entirety of the desired imaged area. However, because of thermal changes, manufacturing defects, shifts of DMDs during installation, and the like, there can develop alignment issues that can impact image quality after fitting into a machine. Thus when using multiple DMDs to form an image on the printing plate, there is a need in the art for compensation or alignment correction of beams to create a stitched image.